Changed Singleton implementation to GCD variant
Changed Singleton implementation to GCD variant

bruceflowers/OHHTTPStubs,sessuru/OHHTTPStubs,ThomWee/OHHTTPStubs,hirohisa/OHHTTPStubs,ThomWee/OHHTTPStubs,liufeigit/OHHTTPStubs,SummerHanada/OHHTTPStubs,ThomWee/OHHTTPStubs,virajs/OHHTTPStubs,oenius/OHHTTPStubs,manuyavuz/OHHTTPStubs,oenius/OHHTTPStubs,hirohisa/OHHTTPStubs,bruceflowers/OHHTTPStubs,kazmasaurus/OHHTTPStubs,SummerHanada/OHHTTPStubs,JinlianWang/OHHTTPStubs,sessuru/OHHTTPStubs,HyprMXMobile/OHHTTPStubs,Labradorom/OHHTTPStubs,JinlianWang/OHHTTPStubs,liufeigit/OHHTTPStubs,HyprMXMobile/OHHTTPStubs,JinlianWang/OHHTTPStubs,kazmasaurus/OHHTTPStubs,virajs/OHHTTPStubs,MaxGabriel/OHHTTPStubs,manuyavuz/OHHTTPStubs,oenius/OHHTTPStubs,virajs/OHHTTPStubs,oenius/OHHTTPStubs,liufeigit/OHHTTPStubs,ThomWee/OHHTTPStubs,SummerHanada/OHHTTPStubs,vladzz/OHHTTPStubs,manuyavuz/OHHTTPStubs,HyprMXMobile/OHHTTPStubs,hirohisa/OHHTTPStubs,tarbrain/OHHTTPStubs,HyprMXMobile/OHHTTPStubs,JinlianWang/OHHTTPStubs,SummerHanada/OHHTTPStubs,kazmasaurus/OHHTTPStubs,tarbrain/OHHTTPStubs,tarbrain/OHHTTPStubs,hirohisa/OHHTTPStubs,tarbrain/OHHTTPStubs,virajs/OHHTTPStubs,bruceflowers/OHHTTPStubs,sessuru/OHHTTPStubs